Drop in Reg Pressure over a string of shots

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L

lbmalleo

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May 9, 2020
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Montana, United States
I’m bugging you guys once again from Patagonia, Argentina with a couple more issues on my Impact MK2.
I am getting a drop in reg pressure of about 10-13 Bar over a 50 shot string. I have a MK2 .25 with Power Block and 72 Power Plenum Upgrade, slug kit. Shooting 34 grain slugs at 945fps (targeted that speed because I found it to be most accurate). Reg is a Huma that I installed along with all the upgrades I stated earlier.
Fill the gun up to 250 Bar, 125 Bar on Reg, I get around 10 shots at around 950fps, the next 10-15 shots the Reg drops to about 120 Bar and velocites around 943 fps, the following shots reg drops to about 117Bar and velocity to around 937 or so while main reg is still around 180-200 Bar. The gun is super consistent but velocities and pressure drop consistently, no peaks or drops.
Reg seems to remain the same while gun is not in use, so it doesn’t seem to have any creep or droop while not in use.
Should I use a heavier hammer to open the valve a bit more or with a little more strength? (I have a thing with asking about hammers because I have no idea when to go heavier!).
I thought about polishing the standard reg that came with the gun and put it back on but I have a feeling its not the reg.
Any suggestions would be greatly appreciated!!

Lucas….again!
 
A change of less than 15fps over the string represents only about a 1.5% extreme spread. That suggests you already have the hammer strike adjusted nicely. In other words, well balanced to the operating pressure. But if you want to fiddle with it, what you’d want to try is a smidge less hammer spring tension to knock down the starting velocity just a bit, which will in turn have a disproportionately smaller effect as the pressure falls.
 
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nervoustrig said:
A change of less than 15fps over the string represents only about a 1.5% extreme spread. That suggests you already have the hammer strike adjusted nicely. In other words, well balanced to the operating pressure. But if you want to fiddle with it, what you’d want to try is a smidge less hammer spring tension to knock down the starting velocity just a bit, which will in turn have a disproportionately smaller effect as the pressure falls.
Click to expand...
Thanks for your reply, I understand that the change is only around 15fps, what I’d like to know is why is my reg pressure dropping consistently as I shoot? Is it the reg? Is there a possible leak somewhere?
I have a Wildcat, its reg is set to 130Bar and it is extremely consistent going back to 130 Bar over a shot string.
L
 
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The nature of the pressure shift is related to the same mechanisms that cause pressure creep. What is happening is, when the reservoir pressure is high, the piston has to put a really strong squeeze against the seat (the plastic disc) to completely arrest the flow of air. Or in other words, to fully halt the pressure from continuing to rise. That extra squeeze is contributed by the approx. 10 bar pressure difference you’re seeing.
Conversely, when the reservoir is at a lower pressure, the seat needs less squeeze to close off. Meaning it will occur at a slightly lower pressure.

The plastic disc takes an imprint and wears over time so you may be able to improve this behavior by replacing the plastic disc, or by simply flipping it over to expose a fresh surface. If you want to take it a step further, make a replacement with a more durable material. I make them from this 0.040” thick Delrin stock:
https://www.mcmaster.com/8738K16

The OEM material is reportedly Delrin as well but it is clearly a much softer composition if you compare them side by side.

Also inspect the tiny orifice on the adjuster screw with the aid of 5x – 10x magnification. If the rim has visible machining marks / striations, carefully dress the surface with ~2000 grit backed by a hard flat surface like a piece of glass or granite. The combination of a good surface finish here and a hard plastic seat will provide a more consistent setpoint (better input regulation) and a longer-lasting seat.
 
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nervoustrig said:
The nature of the pressure shift is related to the same mechanisms that cause pressure creep. What is happening is, when the reservoir pressure is high, the piston has to put a really strong squeeze against the seat (the plastic disc) to completely arrest the flow of air. Or in other words, to fully halt the pressure from continuing to rise. That extra squeeze is contributed by the approx. 10 bar pressure difference you’re seeing.
Conversely, when the reservoir is at a lower pressure, the seat needs less squeeze to close off. Meaning it will occur at a slightly lower pressure.

The plastic disc takes an imprint and wears over time so you may be able to improve this behavior by replacing the plastic disc, or by simply flipping it over to expose a fresh surface. If you want to take it a step further, make a replacement with a more durable material. I make them from this 0.040” thick Delrin stock:
https://www.mcmaster.com/8738K16

The OEM material is reportedly Delrin as well but it is clearly a much softer composition if you compare them side by side.

Also inspect the tiny orifice on the adjuster screw with the aid of 5x – 10x magnification. If the rim has visible machining marks / striations, carefully dress the surface with ~2000 grit backed by a hard flat surface like a piece of glass or granite. The combination of a good surface finish here and a hard plastic seat will provide a more consistent setpoint (better input regulation) and a longer-lasting seat.
Click to expand...
Thanks so much!!
L
 
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Another possible culprit is your regulators refresh rate not being consistent over your pressure range.

At higher pressure, air moves faster, so your regulator may refresh to 125 bar at 230-250 bar, but only to 120 bar around 200-230, with a slope in between.

Could even be an oring that extrudes at 250 bar but not 200 bar, causing the regulator to fill more as it takes a bit more pressure to close the regulator in such event.

Could even be the bellevile stack configuration has a non-linear slope.

-Matt
 
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Stubbers said:
Another possible culprit is your regulators refresh rate not being consistent over your pressure range.

At higher pressure, air moves faster, so your regulator may refresh to 125 bar at 230-250 bar, but only to 120 bar around 200-230, with a slope in between.

Could even be an oring that extrudes at 250 bar but not 200 bar, causing the regulator to fill more as it takes a bit more pressure to close the regulator in such event.

Could even be the bellevile stack configuration has a non-linear slope.

-Matt
Click to expand...
Thanks!
What would be ways to fix it?
 
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Stubbers said:
Process of elimination.

1) Inspect o-rings for any signs of nibbing/extrusion

2) Reduce / restrict regulator refresh orifice somehow

3) Try a different belleville stack configuration and re-tune to your desired pressure.

If none of the above help, then I'd be scratching me head!
Click to expand...
I can’t thank you enough!!
I’ll try all out tomorrow, I already have the reg out!
Saludos,

L
 
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Stubbers said:
Don't forget what Jason suggested if all fails, a soft enough seat material could definitely have the same effect, I just didn't repeat his solution! Best of luck, make sure to report back if you fix it!

-Matt
Click to expand...
So, Jason and Matt, I polished up the reg a bit, did not polish the inner (screwing part) because when I started to pull it out I felt a lot of resistance so I was afraid of damaging an oring. I did polish up with 3000 grit the bottom and sides of the piston and bottom and sides of the reg. I then flipped the delrin piece, I noticed one side of the delrin piece had a pretty harsh round marking on it. Put everything together and started tuning the gun once more.
During tuning I noticed a tiny leak from the main pressure gauge but after a few shots it stopped and didn’t seem to leak again after refilling twice.
Reg is still going down after a string of shots but not as bad as before. Reg is now going down around 5 or 6 Bar, between 123 down to 117 or so over 50 or so shots. Gun is very stable! The last string of shots was short, only 17 shots, 14fps spread and 3.8 STD Dev, I was happy with that so I quit while I was ahead!
I am thinking about changing the delrin piece in the future for a brand new one, I need to make my own!
Thanks again for your help guys,

Lucas
 
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Nicely done, I’m glad to hear you achieved an improvement to the input regulation characteristics, basically cutting it in half it sounds like.

Regarding Matt’s advice about the Bellevilles, the preferred arrangement is a stack that provides you the desired operating pressure somewhere in the middle of the washers’ operating range. Meaning neither very nearly squeezed flat at the highest setpoint the stack can achieve, nor at the other extreme where the stack is barely being compressed at all. If you are interested in exploring that, just let me know. We’ll need a few caliper measurements of the Belleville washers and regulator piston, and then do some simple math.
 
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Stubbers said:
Belleville stacks can alter your 'closing/sealing force' on the seat, just a thought. You can *try* different stack configurations that work better with the delrin seat, or try to move over to peek which should handle the current closing/sealing force better. That's my opinion on the matter. Glad you got results.

-Matt
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Hi Matt, what would “move over to peak” mean or imply? I’ll try alternate stacking configurations, any ideas? I have it stacked the way Huma suggests in their instruction manual. Should I invert it first?
I’ll try to do this next time I take the gun apart so you might get a message out of nowhere asking about it! Hope is doesn’t bother you!
L
 
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PEEK is a hard, tough thermoplastic.

“Alternate stacking configurations” is in reference to perhaps a changing the number of washers, or changing their thickness, or changing from a series arrangement to a series/parallel arrangement. Again, if you’re interested in that question, a good starting point would be the Belleville washer and piston dimensions.
 
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lbmalleo said:
Hi Matt, what would “move over to peak” mean or imply? I’ll try alternate stacking configurations, any ideas? I have it stacked the way Huma suggests in their instruction manual. Should I invert it first?
I’ll try to do this next time I take the gun apart so you might get a message out of nowhere asking about it! Hope is doesn’t bother you!
L
Click to expand...

As Jason stated, PEEK has a higher tensile and yield strength than delrin, so it would be less prone to 'squishing' under pressure. If this is your root cause, this alone should remedy your issue or minimize it further at the very least.

The stack can be arranged quite a few ways to achieve various set point ranges. I am no expert though on these as I don't run them in my reg...

Here is an example from another manufacturer how different stacking arrangements with the same washers work.

1689285809671.png
 
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